This invention relates to a method of manufacturing a semiconductor device and an apparatus for polishing a semiconductor substrate or a film on a semiconductor substrate to planarize the surface of the film.
In the conventional process of forming a semiconductor device such as IC and LSI, a film made of material such as metal, polysilicon or silicon dioxide is formed over the surface of the semiconductor substrate, i.e., semiconductor wafer in which grooves (e.g. trenches, contact holes) are formed, so that the grooves are fully filled with the film material. Subsequently, the surface of the film is planarized. As a method of planarizing the surface of the film, etchback RIE (Reactive Ion Etching) is known.
The etchback RIE, however, requires an additional step of providing etchback photoresist on the surface of the film. This increases the number of the steps of manufacturing the semiconductor device. Furthermore, during the etchback RIE process, the surface of the film may be damaged and, thus, good planarization cannot be easily attained. Moreover, to perform the etchback RIE, a vacuum type apparatus complicated in structure is used. The manufacturing efficiency of the semiconductor device is thus inevitably low. Additionally, it is necessary to use dangerous gas to perform the etchback RIE. The planarization using the etchback RIE has thus various disadvantages.
In view of this, CMP (Chemical Mechanical Polishing) which can be performed by a polishing machine is recently put into practice, instead of the etchback RIE.
The polishing machine used for the CMP process comprising a polishing pad and a wafer holder. The polishing pad is applied with a polishing cloth on the surface thereof and rotated by a motor or the like, and the wafer holder rotatably holds a semiconductor substrate, pressing the substrate against the polishing pad. Generally, when the surface of the film on the substrate is planarized by use of the polishing machine, the substrate is rotated while pressed against the polishing cloth on the rotating polishing pad. The film is thereby polished while slurry supplied on the polishing cloth. The slurry contains as abrasive particles of material such as ceria (cerium oxide), silica (silicon oxide), and silicon nitride (Si.sub.3 N.sub.4). Silicon nitride particles are described in Jpn. Pat. KOKAI Appln. No. 7-317054. When a stopper film is made of silicon nitride and the slurry contains silicon nitride as the abrasive particles, a high etching ratio of the film to be planarized to the stopper film is achieved.
In the conventional polishing process of a semiconductor substrate with use of the CMP polishing machine, the film to be polished (e.g., an oxide film) is polished with the supplied slurry until the thickness of the film decreases to a predetermined value. When the film is planarized to a desired degree, the polishing is stopped. Subsequently, post-polishing is performed while supplying water instead of the slurry in order to remove the foreign articles (abrasive particles, in particular) adhered to the film surface during the main polishing. The post-polishing is performed by supplying pure water or ultra-pure water onto the wafer. Thus, it is called as "DI water-rinsing" or "water-rinsing". The post-polishing process is followed by the cleaning step of cleaning the surface of the film.
If the zeta potential on the surfaces of the particles is opposite in polarity to that on the surface of the polished film, the particles strongly adhere to the surface of the polished film, zeta potential meaning the electric potential existing across the interface of solids and liquids. It is therefore difficult in such cases to remove the particles from the film surface by the conventional post-polishing technique described above. The zeta potential on the surface of the particles adhered to the surface of the silicon semiconductor substrate or the surface of the polished film has a positive or negative polarity. The zeta potential depends on the pH on the surface of the material, as is shown in FIG. 2. In FIG. 2, the zeta potential (mV) on the surface of the material is protted on the ordinate, and the pH on the surface of the material on the abscissa. Curve A shows the zeta potential characteristic of silicon nitride, curve B the zeta potential characteristic of silicon oxide, and curve C shows the zeta potential characteristic of silicon.
In a general polishing machine for performing the CMP process, a slurry most suitable for polishing the material of the film to be polished is used. The films to be polished have different zeta potentials according to the material of the films to be polished. Also, the abrasive particles have different zeta potentials according to the material of the abrasive particles. The adhesion of the particles to the polished film therefore varies in accordance with their zeta potentials. Further, the zeta potential depends on the pH on the material, and thus the number of the particles adhered to the film surface varies in accordance with the pH on the material.